1. Field of the Invention
The present invention relates to the detection of transverse cracking in rail heads and, more particularly, to using a low frequency eddy current system designed to detect transverse cracks in rail heads.
2. Statement of the Problem
It has been reported that 40% of rail failures are due to the occurrence of transverse cracking in the rail head. An important goal in the railroad industry is to detect such transverse cracks before such failure occurs. A transverse crack is a progressive transverse fracture occurring in the head of the rail and propagating under cyclic fatigue.
One conventional nondestructive approach for detecting transverse cracking is the use of conventional ultrasonic methods involving roller search units and contact transducers. The problem with such conventional ultrasonic approaches is that transverse cracks often occur under horizontal cracks (shelling/air gaps) which reflect ultrasound thereby masking the transverse crack. Horizontal cracking is a condition in which the cold worked layer of material above the transverse crack separates and flows over the remaining rail. This xe2x80x9cshellingxe2x80x9d of the rail head is a condition of the contact fatigue generated due to wheel/rail interaction.
A need exists to detect transverse cracks beneath horizontal cracks before the transverse crack grows or fails in service.
In Earnest, Katragadda, Si, and Garcia, xe2x80x9cTransverse Crack Detection in Railroad Heads,xe2x80x9d Oral Presentation at the 6th NDE Topical Conference, Apr. 19-22, 1999, San Antonio, Tex., initial results were disclosed of an experimental low frequency eddy current approach designed to detect transverse cracks in rail heads. While this presentation reported results from static (stationary) testing, it did verify the ability to detect transverse cracks under horizontal cracks in rail head. Horizontal cracks are parallel to the direction of the saturated magnetic field making the probe relatively insensitive to the horizontal cracks and sensitive to any underlying transverse cracks. The static tests not only were able to detect transverse cracks under horizontal cracks, but were able to provide output signals indicating the approximate size of the transverse crack. It was further observed that higher frequencies for the eddy current did not penetrate well into the rail head whereas lower frequencies of 70 to 100 hertz provided deeper penetration. The Earnest et al. presentation speculated that the static feasibility test observed could be implemented into a push cart wherein brushes could be added to the saturating magnetic poles to increase the contact with the rail head.
A need exists to provide a low frequency eddy current system that moves along the rail at a velocity sufficient to detect transverse cracks in rail heads of actual rail tracks, that provides an eddy current probe for such moving inspections, that follows a rail head having a wear profile, that minimizes any lift-off of the probe while moving, and that couples the magnetic saturation to the rail.
A need exists also to reject those signals from the low frequency eddy current probe that are sensed to be transverse cracks, but are from non-relevant indications such as from thermite welds, plant welds, rail end joints, etc.
The present invention solves the aforestated problems by providing a novel method and system for increasing the detection of transverse cracks beneath horizontal cracks in the rail. The novel method and system of the present invention provides a low frequency eddy current system that moves along the rail at a velocity sufficient to detect transverse cracks in rail heads, that provides a low frequency eddy current probe for such moving inspections, that follows the wear profile of the rail head, that minimizes any lift-off of the probe while moving, and that couples the saturation magnet to the rail. The novel method and system of the present invention also utilizes a separate sensor such as a Hall element sensor to distinguish the low frequency eddy current signals, obtained from a transverse crack, from non-relevant indications such as thermite welds, plant welds, rail end joints, etc.
The novel method of the present invention provides moving a transporter on the rail at a velocity, generating a saturation magnetic field into and across the rail head using a DC saturation magnet mounted to the transporter a predetermined distance above the rail head, inductively coupling the opposing pole ends of the DC saturation magnet with the rail head, detecting transverse cracks in the rail head with a low frequency eddy current probe mounted centrally between the opposing pole ends of the DC saturation magnet and over the rail head, applying a force on the low frequency eddy current probe against the rail head as the transporter moves on the rail at the velocity so as to follow the wear pattern of the rail head, and to control the lift-off of the probe from the rail head.
The novel system of the present invention uses a transporter that moves along the track. A toroidal-shaped DC magnet mounted to the transporter with its opposing ends inwardly directed towards each other and aligned over the rail head to increase saturation. An inductive coupling provided between each of the poles and the rail heads increases magnetic saturation of the rail head. A low frequency eddy current probe centrally located between the poles of the toroidal-shaped DC magnet senses the presence of transverse cracks in the rail head. A carriage mounted to the transporter forces the low frequency eddy current probe towards the rail head at the central location. Protective material on the probe abuts the rail head as the transporter moves thereby protecting the low frequency eddy current probe from damage while permitting the probe to follow the wear profile. At least one wheel located on the carriage engages the rail head for controlling lift-off of the low frequency eddy current probe from the rail head.